1. Field of the Invention
The present invention relates to a magnetoresistive element, and to a thin-film magnetic head, a head gimbal assembly, a head arm assembly and a magnetic disk drive each of which includes the magnetoresistive element.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of magnetic disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a structure in which a write head having an induction-type electromagnetic transducer for writing and a read head having a magnetoresistive element (that may be hereinafter referred to as MR element) for reading are stacked on a substrate.
MR elements include AMR (anisotropic magnetoresistive) elements utilizing an anisotropic magnetoresistive effect, GMR (giant magnetoresistive) elements utilizing a giant magnetoresistive effect, and TMR (tunneling magnetoresistive) elements utilizing a tunneling magnetoresistive effect.
Read heads are required to have characteristics of high sensitivity and high output power. As the read heads that satisfy such requirements, GMR heads that employ spin-valve GMR elements have been mass-produced. Recently, to accommodate further improvements in areal recording density, developments have been pursued for read heads employing TMR elements.
A spin-valve GMR element typically includes a free layer, a pinned layer, a nonmagnetic conductive layer disposed between the free layer and the pinned layer, and an antiferromagnetic layer disposed on a side of the pinned layer farther from the nonmagnetic conductive layer. The free layer is a ferromagnetic layer whose direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer whose direction of magnetization is fixed. The antiferromagnetic layer is a layer that fixes the direction of magnetization of the pinned layer by means of exchange coupling with the pinned layer.
Conventional GMR heads have a structure in which a current used for detecting magnetic signals (that is hereinafter referred to as a sense current) is fed in the direction parallel to the plane of each layer making up the GMR element. Such a structure is called a CIP (current-in-plane) structure. On the other hand, developments have been pursued for another type of GMR heads having a structure in which the sense current is fed in a direction intersecting the plane of each layer making up the GMR element, such as the direction perpendicular to the plane of each layer making up the GMR element. Such a structure is called a CPP (current-perpendicular-to-plane) structure. A GMR element used for read heads having the CPP structure is hereinafter called a CPP-GMR element. A GMR element used for read heads having the CIP structure is hereinafter called a CIP-GMR element.
Read heads that employ TMR elements mentioned above have the CPP structure, too. A TMR element typically includes a free layer, a pinned layer, a tunnel barrier layer disposed between the free layer and the pinned layer, and an antiferromagnetic layer disposed on a side of the pinned layer farther from the tunnel barrier layer. The tunnel barrier layer is a nonmagnetic insulating layer through which electrons are capable of passing with spins thereof conserved by the tunnel effect. The free layer, the pinned layer and the antiferromagnetic layer of the TMR element are the same as those of the spin-valve GMR element. As compared with the spin-valve GMR element, the TMR element is expected to provide a higher magnetoresistance change ratio (hereinafter referred to as an MR ratio), which is the ratio of magnetoresistance change with respect to the resistance.
JP 2002-84014A discloses a TMR element having a structure in which a second layer made of a nonmagnetic insulating film is sandwiched between a first layer and a third layer each of which is made of a ferromagnetic metal thin film. This TMR element is designed so that the spin polarization of the second layer is one tenth or less that of each of the first layer and the third layer. The second layer is made of a ZnOx thin film (x=0.95 to 1.05) as an insulating film.
JP 2006-86476A discloses a magnetic recording element including: a free layer whose direction of magnetization is changed by the action of spin-polarized electrons; a pinned layer whose direction of magnetization is fixed; and an intermediate layer made of a nonmagnetic material and provided between the pinned layer and the free layer. JP 2006-86476A lists a nonmagnetic metal, an insulating material and a semiconductor material as the material of the intermediate layer. In this magnetic recording element, the direction of magnetization of the free layer is changed by injecting spin-polarized electrons into the free layer.
JP 2006-54257A discloses a CPP-GMR element including: a magnetization pinned layer whose direction of magnetization is pinned; a magnetization free layer whose direction of magnetization changes in response to an external magnetic field; and a spacer layer disposed between the magnetization pinned layer and the magnetization free layer, the spacer layer including an insulating layer and current paths penetrating the insulating layer. In this CPP-GMR element, the insulating layer is made of Al2O3, for example, and the current paths are made of Cu, for example. Such a CPP-GMR element is called a current-confined-path CPP-GMR element, for example.
JP 2003-8102A discloses a CPP-GMR element including: a magnetization pinned layer whose direction of magnetization is pinned; a magnetization free layer whose direction of magnetization changes in response to an external magnetic field; a nonmagnetic metal intermediate layer provided between the magnetization pinned layer and the magnetization free layer; and a resistance adjustment layer provided between the magnetization pinned layer and the magnetization free layer and made of a material containing conductive carriers not more than 1022/cm3. JP 2003-8102A discloses that the material of the resistance adjustment layer is preferably a semiconductor or a semimetal.
Properties of ZnO films such as resistivity are disclosed in “Technology of Transparent Conductive Film”, edited by Japan Society for the Promotion of Science, the 166th Committee on Transparent Oxides and Photoelectronic Materials, p. 168.
To use a TMR element for a read head, it is required that the TMR element be reduced in resistance. The reason for this will now be described. Improvements in both recording density and data transfer rate are required of a magnetic disk drive. Accordingly, it is required that the read head exhibit a good high frequency response. However, a TMR element with a high resistance would cause a high stray capacitance in the TMR element and a circuit connected thereto, thereby degrading the high frequency response of the read head. For this reason, it is required that the TMR element be reduced in resistance.
To reduce the resistance of the TMR element, it is typically effective to reduce the thickness of the tunnel barrier layer. However, an excessive reduction in the thickness of the tunnel barrier layer would cause a number of pinholes to develop in the tunnel barrier layer, resulting in a shorter service life of the TMR element. In addition to this, a magnetic coupling may also be established between the free layer and the pinned layer, resulting in deterioration of characteristics of the TMR element such as an increase in noise or a reduction in MR ratio. Here, noise that occurs in read heads is referred to as head noise. Head noise that occurs in a read head employing a TMR element includes shot noise which is a noise component that will not be generated in a read head employing a GMR element. For this reason, a read head employing a TMR element has a problem that it develops greater head noise.
JP 2002-84014A discloses that, by employing a ZnOx thin film (x=0.95 to 1.05) to form the nonmagnetic insulating film as the second layer, it is possible to enhance tolerance to variations of the insulating film and to thereby provide improved reliability of the TMR element. However, JP 2002-84014A gives no consideration to a reduction in resistance of the TMR element.
On the other hand, a CPP-GMR element has a problem that it cannot provide a sufficiently high MR ratio. This is presumably because spin-polarized electrons are scattered at the interface between the nonmagnetic conductive layer and a magnetic layer or in the nonmagnetic conductive layer.
Additionally, a CPP-GMR element is small in magnetoresistance change amount because of its low resistance. Accordingly, in order to obtain higher read output power using a CPP-GMR element, it is necessary to apply a higher voltage to the element. However, the application of a higher voltage to the element would raise the following problems. In a CPP-GMR element, a current is fed in the direction perpendicular to the plane of each layer. This would cause spin-polarized electrons to be injected from the free layer into the pinned layer or from the pinned layer into the free layer. These spin-polarized electrons would produce torque in the free layer or the pinned layer to rotate the magnetization thereof. This torque is herein referred to as spin torque. The spin torque is proportional to the current density. As the voltage applied to the CPP-GMR element is increased, the current density will also increase, thereby causing an increase in the spin torque. An increase in the spin torque would lead to a change in the direction of magnetization of the pinned layer.
The magnetic recording element disclosed in JP 2006-86476A is designed to make use of the aforementioned spin torque to change the direction of magnetization of the free layer. However, as described above, for a CPP-GMR element used for a read head, an increase in spin torque is undesirable because it would change the direction of magnetization of the pinned layer to thereby cause deterioration of the characteristics of the read head.
A current-confined-path CPP-GMR element such as the element disclosed in JP 2006-54257A allows the resistance and magnetoresistance change amount thereof to be greater as compared with a typical CPP-GMR element. However, the current-confined-path CPP-GMR element has the following problems. That is, the insulating layer in the spacer layer of the current-confined-path CPP-GMR element is formed through oxidation treatment, for example. In this case, since the oxidation state of the insulating layer greatly varies, it is difficult to form the spacer layer with stability. Thus, the current-confined-path CPP-GMR element will suffer greater variations in characteristics thereof. Furthermore, like a typical CPP-GMR element, the current-confined-path CPP-GMR element cannot provide a sufficiently high MR ratio because the spin-polarized electrons are scattered in the spacer layer.
The CPP-GMR element disclosed in JP 2003-8102A allows the resistance and magnetoresistance change amount thereof to be greater as compared with a typical CPP-GMR element. JP 2003-8102A discloses that, in order to prevent relaxation of spins in the resistance adjustment layer, it is preferred that the resistance adjustment layer be smaller in thickness and that the thickness be 1 nm or smaller. However, JP 2003-8102A does not disclose a preferable range of the resistance of the CPP-GMR element or that of the thickness of the resistance adjustment layer to be employed in order to provide a high MR ratio while suppressing noise and the effects of spin torque.